Imagine, a mountain of ice 100 feet tall moving about 2 inches a year. As that great wall of ice retreated it carved out a number of huge lakes.
One of those lakes is Lake Michigan. Lake Michigan sits between Illinois Wisconsin, Michigan, and Indiana and as the glacier retreated, it grounds lots of Rock into Sand.
There are Dunes all around the southern shore of Lake Michigan, a great place to go and relax in the sand by the shore of Lake Michigan!